The present invention relates generally to disposable food service articles and more particularly to compartmented food service articles thermoformed from plastic sheet by way of plug-assist thermoforming.
Plug-assist thermoforming is often desirable, especially to aid in the redistribution of material into a mold. Typically, a mechanically driven plug is used to prestretch the plastic sheet into a female mold cavity or around mold features. Vacuum and/or pressure is then applied to pull and/or push the sheet against the mold surface where it is cooled to set the shape. The heat softened plastic sheet can either be formed up (plug on bottom/mold on top) or formed down (plug on top/mold on bottom). Numerous other variations of plug-assisted thermoforming are also possible as discussed by Professor Throne. A detailed discussion of plug-assist thermoforming is set forth in a book entitled Thermoforming by James L. Throne (1987).
As noted on pg. 207 of Thermoforming: xe2x80x9cThe plug design parameters include the shape of the plug tip, the plug penetration depth (relative to the cavity depth) and the plug diameter (relative to the cavity diameter). The plug surface temperature and coefficient of friction (between the plug surface and the stretching sheet) are also considered design factors.xe2x80x9d
Plugs can be produced from wood, syntactic foam, aluminum, etc. Plugs are frequently heated or cooled. Syntactic foam plugs typically are not heated or cooled since they are insulators. Syntactic foam is commonly used since it is easily machined and does not instantaneously chill the plastic sheet upon contact. Plugs may be designed with a hollow center bottom so they do not cool the bottom material and can more easily redistribute the material towards the corners where sheet thinning will often occur. Internal mold corners and edges will typically be substantially thinner than the original plastic sheet thickness from the sheet stretching process as detailed in chapters 4 and 7 of Thermoforming. 
In order to overcome the problems of sheet thinning and to assist the formation of shaped articles, a variety of plug-assist techniques have been employed. See, for example, U.S. Pat. No. 5,641,524 of Rush et al. where an apparatus useful for thermoforming a disposable cup is illustrated.
Some compartmented plastic articles can be produced without any plug-assist. Large generous internal corners and edges, gentling sloping side angles and shallow depths are often features designed into the products to minimize sheet thinning. Higher caliper sheet can also be used to compensate for sheet thinning. This increases the cost for the disposable plastic products and may not be an acceptable alternative.
Plug-assist forming can be applied to aid in the stretching and material distribution for the compartmented plastic product. One option for a plug design may be an offset of the mold profile and rib geometry (angle, height, etc.) stroked to within xe2x85x9 to xc2xc inch from the mold bottom and/or rib sidewall and top. Another option is to employ a plug with larger external radii between the plug bottom and relieved offset rib profile. Such alternatives do not provide the advantages of the present invention as discussed hereinafter.
The plug design of the present invention typically has a near vertical rib relief sidewall (xcx9c90 degrees to plug bottom), and a near sharp/small external radii between the plug bottom and near vertically relieved rib sidewall. It was determined through experimentation/prototype production, that this design provided the least amount of rib to plate bottom corner thinning compared to two other plug designs. The preferred compartmented plate design has near sharp rib to bottom and rib to profile junctions where excessive thinning can occur. The excessive thinning may allow food fluids to seep through due to plastic xe2x80x9cpinholingxe2x80x9d.
Specific improvements of the present invention include positioning the near sharp external radii between the plug bottom and vertical relief areas near vertically in-line with the mold rib to bottom and rib to profile near sharp junctions. It is possible to extend the plug along the plate mold sidewall profile to reduce sheet thinning at the transition of the rib to sidewall profile. The bottom portions of the plug may be concave hollowed such as not to contact/cool the sheet and to allow more stretching into the corner near sharp junctions to further reduce bottom thinning.
The plug design of the present invention provides utility for compartmented disposable food serving containers regardless of their shape. The overall container shape could be round, oval, square, rectangular or polyhexal with rounded corners. Containers with any number of compartments with ribbed dividers greater than two can be formed with this plug design. This design is most useful when the rib to bottom and rib to profile junctions are near sharp. Any form of plug-assisted thermoforming can be practiced with the described plug design and positioning technique. The unique plug design allows the use of low caliper, lighter weight plastic sheet to produce containers with near sharp rib edge designs and minimizes the severe corner thinning that often occurs.
Salient features of the plug design which provide the thermoforming and container benefits in particular embodiments are:
1. Near Vertical Rib Relief
2. Near Sharp/Small Radii External Rib Relief Edges
3. Rib Relief Edges Positioned Near Vertically Aligned Above Mold Rib to Bottom and Rib to Profile Junctions
4. Plug Closure to Approximately {fraction (1/16)}xe2x80x3 From Product Profile and/or Rib
In another aspect of the invention, there is provided compartmented disposable food serving article having a substantially planar bottom portion and a plurality of rib portions configured so as to segment the article into a plurality of food serving compartments, wherein the rib portions are substantially angularly joined to the bottom portion and wherein the food serving article is thermoformed utilizing a plug-assisted thermoforming process. The process includes: positioning a softened plastic sheet to be in close proximity to a thermoforming mold having a substantially planar mold surface and a plurality of rib portions projecting axially from the substantially planar mold surface. Typically, the heat softened plastic sheet is perimeter or individually clamped around the thermoforming mold or molds to create a seal. The rib portions define a plurality of angular junctions with the substantially planar mold surface. The second step of the process is thermoforming the disposable compartmented food service container, wherein a plug-assist member is applied to the softened plastic sheet to urge the sheet toward conformity with the thermoforming mold. The plug-assist member includes a plurality of rib relief portions positioned, configured and dimensioned to fit about the rib portions of the mold in a forming operation at a forming clearance distance of about 0.125 inches or less from the mold. The plug is also provided with a plurality of substantially vertical wall rib relief portions configured and dimensioned to be substantially vertically aligned with the angular junctions of the rib portions of the mold with the substantially planar mold surface. The vertical wall portions are adjacent a plurality of angular corner portions transitioning between the vertical wall rib relief portions of the plug and a bottom surface of the plug.
The compartmented disposable food serving article generally has a wall thickness of from about 10 to about 80 thousandths of an inch (xe2x80x9cmilsxe2x80x9d) with a wall thickness of from about 15 to about 25 mils being more typical.
The articles may be formed from a variety of filled or unfilled polymers such as addition polymers or condensation polymers. Polymers employed may be those listed on page 53 of Thermoforming noted above, particularly ABS; Acronitrile; Acetate, PMMA; Acrylic/PVC; Butyrate; PC; PET; Polyethersulfone; 20% GR PES; HDPE; Propionate; PP; 40% GR PP; P-Sulfone; P-Styrene; PTFE/FEP; Rigid PVC; Mod. PPO. Vinyl-type thermoplastics, listed on page 41 of Thermoforming are frequently preferred, especially: Polyethylene; Polypropylene; Polybutene; Polybutadiene (divinyl); Polyvinyl chloride (PVC); Polyvinyl fluoride (PVF); Polyvinyl dichloride (PVDC); Polyvinylidene fluoride (PVF2); Polytetrafluoroethylene (PTFE); Polystyrene (PS); Polyvinyl Alcohol (PVOH); Polyvinyl Acetate (PVAc); Polymethylmethacrylate (PMMA); Polyacrylonitrile. Particularly preferred plastics useful in connection with the present invention include polystyrene, nylons, polypropylene, acrylic polymers and polyethylene terephthalate (xe2x80x9cPETxe2x80x9d).
Particularly preferred compositions for forming food serving plates, trays, and the like, in accordance with the invention include mineral-filled polypropylene materials.
Particularly preferred articles may have a wall thickness of from about 10 to about 80 mils, typically from 15 to 25 mils, and consist essentially of from about 40 to about 90 percent by weight of a polypropylene polymer, from about 10 to about 60 percent by weight of a mineral filler, from about 1 to about 15 percent by weight polyethylene, from about 0.1 to about 5 weight percent titanium dioxide and may optionally include a basic organic or inorganic compound comprising the reaction product of an alkali metal or alkaline earth element with carbonates, phosphates, carboxylic acids as well as alkali metal and alkaline earth element oxides, hydroxides, or silicates and basic metal oxides, including mixtures of silicon dioxide with one or more of the following oxides: magnesium oxide, calcium oxide, barium oxide, and mixtures thereof. The basic organic or inorganic compound preferably comprises calcium carbonate which is present in an amount of from about 5 to about 20 weight percent. Polyethylene is generally present from about 2.5 to about 15 percent by weight, but more typically present from about 4 to about 5 weight percent. Titanium dioxide may be present from about 0.1 to about 3 weight percent; however titanium dioxide is usually present from about 0.25 to about 2 percent by weight, and titanium dioxide is present in an amount of at least about 0.5 percent by weight in particular embodiments.
A particularly preferred mineral filler is mica and a particularly preferred polypropylene polymer is isotactic polypropylene which has a melt index of from about 0.3 to about 4 and most preferably has a melt flow index of about 1.5. The polyethylene employed in some embodiments may be HDPE or LLDPE.
A particularly preferred article which may be produced in accordance with the present invention is a compartmented food serving plate with an asymmetric rib design described in detail hereinafter. These plates include a substantially planar bottom portion, a sidewall portion extending about the periphery of the bottom portion and projecting upwardly therefrom, a flange portion extending outwardly from the sidewall portion and a divider portion configured so as to segment the plate into three compartments of differing volume. The divider portion has a central junction portion and three ribs extending outwardly therefrom, a first rib, a second rib and a third rib; each of the ribs projects upwardly from the bottom portion and extends from the central junction portion to the sidewall portion, wherein each of the ribs is substantially angularly joined to the bottom portion and the sidewall portion and wherein a first included angle between the first and second ribs differs from a second included angle between the second and third ribs and a third included angle between the first and third ribs differs from the first and second included angles. In general, the first and second included angles, in the aggregate, total more than about 200 degrees. More typically, the first included angle is from about 90 to about 110 degrees and the second included angle is from about 115 to about 135 degrees. The third included angle is typically from about 125 to about 145 degrees. In particularly preferred embodiments, each of the first, second and third ribs has a substantially flat top portion, which is angularly joined to a pair of upwardly projecting rib walls. The ratio of the diameter of the plate to the widths of the substantially flat top portions of the ribs is generally from about 25 to about 110, with from about 65 to about 90 being preferred.
In yet still another aspect of the present invention, there is provided a method of making a disposable, compartmented food service article by way of plug-assisted thermoforming comprising: positioning a softened plastic sheet to be formed in proximity to a thermoforming mold having a substantially planar mold surface and a plurality of rib portions projecting axially from the substantially planar mold surface, the rib portions defining a plurality of angular junctions with said substantially planar mold surface; and thermoforming the disposable compartmented food service container, wherein a plug-assist member is applied to said softened plastic sheet to urge said sheet toward conformity with the thermoforming mold, said plug-assist member comprising a plurality of rib relief portions positioned, configured and dimensioned to fit about the rib portions of the mold in a forming operation at a forming clearance distance of about 0.125 inches or less from the mold, the plug being provided with a plurality of substantially vertical wall rib relief portions configured and dimensioned to be substantially vertically aligned with the angular junctions of the rib portions of the mold with the substantially planar mold surface. The vertical wall portions are adjacent a plurality of angular corner portions transitioning between the vertical wall rib relief portions of said plug and a bottom surface of the plug. Typically, the plug-assist member is positioned, configured and dimensioned to fit about the rib portions of the mold at a forming clearance distance of about 0.075 inches or less; whereas, the substantially vertical wall rib relief portions of the plug-assist member are vertically aligned with the angular junctions of the rib portions of the mold with said substantially planar mold surface within a transverse distance of about 0.1 inch and preferably within a transverse distance of about 0.075 inches. Typically, the corner portions of the plug-assist member have a radius of curvature of from 0 to about 0.125 inches and commonly from 0.03 inches to about 0.09 inches. The substantially vertical wall rib relief portions of said plug-assist member are substantially vertically aligned with respect to said substantially planar surface mold surface within an angle of from about 70xc2x0 to about 110xc2x0 in most embodiments. Further aspects of the present invention will be appreciated from the appended drawings and following detailed description.